1. Field of the Invention
This invention relates to a method and an apparatus for co-extruding a multi-layered flexible tube. More specifically, this invention relates to a method and an apparatus for co-extruding a flexible tube having at least a rubber inner layer and a plastic outer layer for use in transporting fuels and other hazardous fluids.
2. Description of Related Art
In order to meet U.S. EPA standards for automotive vehicle emissions, any conduit serving as a fuel transporting hose must be sufficiently impermeable to hydrocarbon vapors emitted by various fuels. A typical fuel-carrying hose comprises one or more layers of rubber or plastic materials or a combination thereof. One such composite fuel and vapor barrier tube is disclosed in PCT Application Ser. No. PCT/US94/07375, filed Jun. 30, 1994, Internaitonal Publication No. WO96/00657, Published 11 Jan. 1996, and is incorporated herein by reference. The application discloses a composite tube comprised of an inner layer of a rubbery material such as FKM, a terpolymer of tetrafluoroethylene (TFE), hexafluoropropylene (HFE) and vinylidine chloride (VF.sub.2) and at least one outer layer of a plastic material such as THV, also a terpolymer of TFE, HFE and VF.sub.2. Multi-layer tubular structures such as this are generally co-extruded in a continuous process to provide a resilient and tubular article.
Several prior art references disclose extrusion processes which produce a multi-layer tubular article which may or may not have an intervening reinforcing layer.
U.S. Pat. No. 4,422,911 to Phillips, issued Dec. 27, 1983, discloses a method of making an extruded plastic tube having inner and outer portions. The reference discloses that a heater is typically provided concentrically around the main body of the extruder to control the temperature of the extruded plastic material during the extrusion process.
The European Patent Application Publication No. 0 551 094 discloses a method of making a fluoropolymer composite tube by co-extruding a conductive fluoropolymer with a non-conductive fluoropolymer, treating the outer layer of the non-conductive fluoropolymer with a corona discharge and crosshead extruding a plastic polyamide layer onto the non-conductive, treated fluoropolymer layer.
U.S. Pat. No. 4,362,488 to Casals et al., issued Dec. 7, 1982, discloses an extruder for the continuous manufacture of reinforced tubing including a tubular spacer member which cooperates with both an inner tubular member and an intermediate tubular member to define an annular passage through which the material to be extruded passes during the extrusion process. The inner and intermediate tubular members include interior passages for circulation of a temperature-regulating fluid therethrough such that the extruder head is maintained at a proper temperature throughout the extrusion operation.
One problem not solved by the prior art is the difficulty in co-extruding a rubbery material with a plastic material due to the different extrusion temperature requirements of each during the process. A rubber material such as FKM is generally extruded at a temperature of perhaps 185.degree.-220.degree. F. whereas a plastic material such as THV must be extruded at a much higher temperature of about 480.degree. F. If the rubber layer is subjected to temperatures of 480.degree. F., the rubber layer will be scorched and, thus, the end product will have undesirable properties.